Fluorinated sulfinates have utility in fluoropolymer and hydrocarbon processing. Methods for the synthesis of fluorinated sulfinates and their use as intermediates have been widely reported in the literature. For example, highly fluorinated alkane sulfinates can be prepared from the corresponding perfluoroalkanehalides via a dehalogenation and sulfination reaction, as reported in C. M. Hu, F. L. Quing, and W. Y. Huang, J Org Chem, 1991, 2801-2804 and W. Y. Huang, Journal of Fluorine Chemistry, 58, 1992, 1-8. Several reagent systems have been developed for use in this reaction, such as sulfite plus an oxidant, hydroxymethane sulfinate, thiourea dioxide and sodium dithionite. The use of sodium dithionite as dehalogenating and sulfinating reagent has also been reported by W. Y. Huang, B. N. Huang and W. Wang in Acta Chim. Sinica (Engl. Ed.), 1986, 178-184, and Acta Chim. Sinica (Engl. Ed.), 1986, 68-72. The later publication discloses that the reaction with an aqueous solution of the sodium dithionite is too slow for reactions involving water-insoluble perfluoralkyl bromides, and that cosolvents are needed to improve the mutual solubility of the various reactants and permit completion of the reaction within 30 to 35 hours. Mentioned cosolvents include acetonitrile, glycol and diethylene glycol.
In another example, F. H. Wu and B. N. Huang, Journal of Fluorine Chem, 67, 1994, 233-234 reported that if DMF, acetonitrile or alcohols are used as cosolvent, both polyfluoroalkyl iodides and polyfluoroalkyl bromides will react with sodium disulfite in neutral aqueous solution to give the corresponding sulfinates in good yield. In a similar manner, CF3CCl3 reacts with sodium disulfite to give the corresponding sodium sulfinate. A disadvantage of preparing fluorinated sulfinates starting from the corresponding fluorinated iodide or bromide is that the resulting reaction product contains a large amount of by-products, particularly, inorganic salts which typically must be removed from the sulfinate.
Alternative processes for the preparation of fluorocarbon sulfinates have also been disclosed, for example, in U.S. Pat. No. 3,420,877. This particular preparation involves reacting perfluoroalkyl sulfonyl fluoride with an alkali metal sulfite or alkaline earth sulfite in an aqueous medium containing from about 10 to about 50 weight percent of a dissolved polar, inert organic solvent selected from the group consisting of dioxane, dimethoxyethane, di-n-butyl ether, tetrahydrofuran, and diethylene glycol diethyl ether. This process generally does not result in large amounts of salts that need to be removed from the resultant product, but requires use of a cosolvent that may be toxic and may have a negative impact on processes in which the sulfinate is ultimately employed, e.g., free-radical polymerization reactions. Reduction of these fluorinated sulfonyl fluorides using NH2NH2 are also known to make the corresponding sulfinates. However, all known processes are limited to making mono-sulfinates and di-sulfinates.
There continues to be a need for a process for highly fluorinated sulfinic acid oligomers and co-oligomers and salts thereof and methods of preparing them that do not require the use of toxic solvents and preferably does not require further processing or purification of the resulting reaction mixture. It is further desirable to have a favorable yield of the highly fluorinated sulfinic acid oligomers and co-oligomers and salts thereof.